1. Field of the Invention
The present invention relates to a microwave oven magnetron having a choking structure and, more particularly, to an improvement in its high frequency output section.
2. Description of the Related Art
A conventional microwave oven magnetron has a structure shown in FIG. 1. An oscillator body 21 of the magnetron shown in FIG. 1 comprises an anode cylinder 22, a plurality of anode vanes 23 fixed inside the anode cylinder 22 and constituting part of a cavity resonator, strap rings 24 for electrically connecting the anode vanes 23, a filament cathode 25 arranged along the axis of the anode cylinder 22, end shields 26 formed at both ends of the filament cathode 25, and pole pieces 27 and 28 fixed to open end portions of the anode cylinder. A cylindrical output section metal vessel 29 is fixed in the anode cylinder 22. An output section ceramic cylinder 31 of a high frequency output section 30 is fixed in the metal vessel 29. A ring 32 for sealing the output distal end portion is arranged inside the high frequency output section 30. A metal exhaust tube 33 is hermetically bonded to the ring 32, and an output section metal cap 34 fits on the ring 32. An output antenna lead 35 is arranged inside the high frequency output section 30. That is, one end portion 35a of the antenna lead 35 passes through a through hole 27a of a pole piece 27 connected to one of the vanes, and then passes through the metal vessel 29 and the ceramic cylinder 31. A distal end portion 35b is clamped and hermetically sealed by the metal exhaust tube 33. A ring-like permanent magnet 36 coaxially surrounds the metal vessel 29 and is magnetically coupled by a ferromagnetic yoke 37. A ferromagnetic thin plate 38 is interposed between the ferromagnetic yoke 37 and the magnet 36, and a net-like conductive gasket 39 is fitted in the inner surface of the ferromagnetic yoke 37. A small-diameter metal cylinder 40 is arranged in the lower end portion of the ceramic cylinder 31, and a large-diameter metal cylinder 41 is arranged to surround the cylinder 40. The metal cylinder 41 is brazed to the distal end portion of the metal vessel 29. A distal end 41a of the metal cylinder 41 holds the inner circumferential portion of the gasket 39. With this structure, a 1/4 wavelength choking groove C2 for chocking the second harmonic wave and a groove C4 for choken the fourth harmonic wave are formed in a discharge tube portion. The metal vessel 29 and two metal cylinders 40 and 41 inside the vessel 29 constitute a groove C3 for choking a third harmonic wave and a groove C5 for choking a fifth harmonic wave. The choking metal cylinders 40 and 41 are formed by ferromagnetic thin-walled cylinders made of iron or an iron alloy. The metal cylinder 40 has an inner diameter D1 smaller than an inner diameter D2 of the ceramic cylinder 31 and has a size smaller than 1/2 of the fifth harmonic wavelength so as to obtain a sufficient choking action. In this magnetron, a fundamental wave having a frequency of, e.g., 2,450 MHz is efficiently radiated from the output section. However, external radiation of the harmonic components is suppressed by the choking action of each 1/4 wavelength choke.
In order to obtain the choken of the harmonic components of higher orders such a the fifth harmonic wave, the inner diameter of the harmonic choking metal cylinder 40 must be reduced to a given degree. When the inner diameter is so reduced, a distance s between the choking metal cylinder 40 and the antenna lead 35 passing therethrough is inevitably reduced. When a high frequency voltage which is applied between the metal cylinder 40 and the antenna lead 35, due to reflected wave produced by an impedance of the microwave oven, reaches a predetermined range, high frequency discharge or RF discharge may occur. In the worst case, when rotation of a stirrer fan is stopped due to a certain cause during operation of the microwave oven and an object to be heated as a high frequency load is almost or perfectly absent so that high frequency reflection onto the magnetron may exceed a standing wave ratio (VSWR) of 30, a discharge is generated between the antenna lead and the harmonic choking metal cylinder. In the extreme case, the antenna lead 35 or the choking metal cylinder 40 is partially heated by the high frequency discharge and may be melt. When part of the antenna lead or the choking metal cylinder melts, a gas discharge may be locally generated by a gas generated upon melting of such a member. In addition, the gas discharge may further cause a high frequency short circuit and reflection. Continuous discharges may then occur in the output section or decisive melting of or damage to the components may occur.
These high frequency discharges may be estimated to be multipactor discharge phenomena in most cases. DC magnetic fluxes leaking from the permanent magnet 36 extend parallel to a tube axis Z, as shown by reference symbol F in FIG. 2 in a space between the antenna lead and the harmonic choking metal cylinder. These magnetic fluxes are almost symmetrical about the tube axis. These metal components have a secondary electron emission rate of 1 or more. When free electrons and the like collide against the antenna lead or the inner surface of the choking metal cylinder, many secondary electrons are generated. These electrons are accelerated or decelerated by a high frequency electric field generated between the antenna lead and the metal cylinder. When electrons emitted from one of these conductors encounter a high frequency accelerating electric field, they are accelerated, and the accelerated electrons collide against the other conductor, thereby emitting a larger number of secondary electrons. In this state, when the high frequency electric field is inverted to form an electric field for accelerating these secondary electrons in a direction to the source conductor, i.e., one conductor, the secondary electrons are accelerated and collide against the source conductor, thereby emitting a larger number of secondary electrons. In this manner, when the electrons are synchronized with the high frequency electric field, the secondary electrons are exponentially increased, and their energy is also increased. Then, the conductors are heated and may be melted. This phenomenon is a double-side multipactor discharge phenomenon.
A second problem occurs due to, the electron rotation caused by a DC magnetic field present in this region. When the period of the high frequency electric field i synchronized with the rotation period, secondary electrons e are cumulatively generated, as illustrated in FIG. 3. The collision energy of these secondary electrons causes abrupt heating of the metal cylinder material, and the metal cylinder may. This phenomenon is called a one-side multipactor discharge phenomenon.